...and I was one of them! The article is only in German--but to sum it up, 50,000 people were divided into four parts and sang the song "Are you sleeping, Brother John?" (in German) in rounds (or canons as the Germans call it). You can listen to the official recording below by clicking on the link or watch the home video I made of it directly underneath. If you watch the video, you can see the blue light of the director waving in the air on a huge tower in the middle.
A more official sound recording of the event (also in German but if you skip towards the end you can actually hear the song Bruder Jakob):